1. Field of the Invention
This invention relates to an apparatus inspection apparatus for determining the presence and location of a tiny foreign particle such as dust, and in particular to an inspecting apparatus for determining the presence and location of a defect such as a tiny foreign particle adhering to the surface of a photomask, a reticle or a semiconductive wafer used in the manufacturing process of integrated circuits or of a pellicle or the like for protecting the photomask or the reticle from dust or the like.
2. Related Background Art
In the photolithography process which is one of the manufacturing processes of integrated circuits, transfer of a circuit pattern to a semiconductive wafer is effected by the use of a reticle or a photomask (hereinafter simply referred to as the reticle).
In this case, if a foreign particle such as dust adheres to the reticle, it presents itself as a defect of the circuit pattern when the transfer thereof to the semiconductive wafer is effected, and this causes a reduction or the like in yield.
A foreign particle inspecting apparatus for determining whether a tiny foreign particle adheres to the surface of a reticle or the like is known from U S Patents Nos. 4,468,120 and 4,610,541. This foreign particle inspecting apparatus is designed such that light is applied to an object to be inspected and scattered light from a foreign particle is detected by the use of photoelectric detector means, whereby the information regarding the position and size of the foreign particle on the surface of the reticle or the like is obtained, and such apparatus is useful to prevent an inconvenience such as the above-mentioned reduction in yield.
Also, recently, as a method of preventing a tiny foreign particle from adhering to the surface of a substrate such as a reticle, it has been the practice to mount a transparent thin film called a pellicle (a foreign particle adherance preventing film) on the surface of the reticle or the like. That is, the pellicle is mounted on that reticle or the like through a support frame to cover the surface of the reticle or the like with the pellicle, thereby preventing a foreign particle from directly adhering to the reticle or the like. Where projection exposure is effected by an exposure apparatus by the use of the reticle or the like covered with the pellicle, even if a foreign particle adheres to the surface of the pellicle, the image of the foreign particle is not focused and formed on the surface of an object to be projected, i.e., a semiconductive wafer, and thus, such image of the foreign particle is not transferred.
However, where the foreign particle adhering to the surface of the pellicle is relatively large, exposure irregularity may occur at a position on the surface of the semiconductive wafer which corresponds to the foreign particle. Also, a foreign particle adhering to the lower surface of the pellicle, i.e., that surface of the pellicle which is adjacent to the reticle, even if not so large as to cause exposure irregularity, may leave the surface of the pellicle and adhere to the surface of the reticle or the like, and if such foreign particle adheres to the surface of the reticle, the image of the foreign particle will be transferred to the semiconductive wafer.
Accordingly, even where a pellicle is used, it is necessary to determine the position and size of a foreign particle adhering to the pellicle and further, it is also necessary to discriminate whether the foreign particle adheres to the upper surface of the pellicle (the surface opposite to the reticle) or the lower surface of the pellicle (the surface which is adjacent to the reticle).
Where as described above, a pellicle is used to cover the reticle or the like, it is necessary to determine not only the position and size of a foreign particle adhering to the pellicle, but also to which of the upper surface or the lower surface (i.e., the outer surface or the inner surface) of the pellicle the foreign particle adheres, before as well as after the pellicle is mounted on the reticle or the like. However, the foreign particle inspecting apparatus according to the prior art has suffered from the problem that although it can know the position and size of the foreign particle on the surface of the pellicle, It cannot discriminate to which of the upper surface of the pellicle (the surface opposite to the reticle or the like or the lower surface of the pellicle (the surface which is adjacent to the reticle) the foreign particle adheres, because the pellicle is very thin.